You are exactly right. If the GOP made this the issue in November after coming out for enforcement of current law, they'd win big.
But they won't becasue the MSM will call them racist.
You're right about the national party as a whole. It's scared of its own shadow. But individual incumbents and challengers can and often will use the issue, because when it comes down to individual self-interest, most people will do what they have to.